1. Field of the Invention
This invention relates generally to electron emissive materials and more particularly to mixed rare earth hexaborides.
2. Description of the Prior Art
Many modern instruments, such as scanning electron microscopes, electron beam exposure systems, cathode ray tubes and magnetrons used in radar require high intensity electron beams. These beams must often be both small and dimensionally stable. The material that is the source of an electron beam with these properties should have a high melting temperature, low vapor pressure, chemical stability at elevated temperatures as well as a low work function and a large value for the coefficient, A, in the Richardson-Dushman equation. The electron emissive material used to generate electron beams has generally been tungsten or thoriated tungsten. While perfectly satisfactory for many purposes, the high temperatures, normally about 2500 degrees C., required for satisfactory current densities lead to dimensional instabilities of the electron beam and shortened emitter lifetimes which are commercially undesirable.
Much effort has been spent looking for electron emissive materials having properties superior to those of tungsten and thoriated tungsten electron sources. Lafferty's work, Journal of Applied Physics 22, 299 (1951), demonstrated that single rare earth hexaborides are attractive candidates because of their relatively low work functions and generally high melting temperatures. Of the single rare earth hexaborides, lanthanum hexaboride, LaB.sub.6, has emerged as the most attractive electron emissive material. As shown by Broers, Reviews of Scientific Instruments 40, 1040 (1968), and Ferris et al U.S. Pat. No. 4,054,946, LaB.sub.6 has a long lifetime, a high melting temperature, a low work function and is chemically stable at the elevated temperatures necessary for thermionic emission.
The properties of the single rare earth hexaboride systems have prompted work to find better materials in other hexaboride systems. One direction that has been taken is the investigation of rare earth hexaborides that are doped or mixed with other materials or compounds. Several mixed hexaboride systems have been investigated and include (NaLa)B.sub.6, Journal of Applied Chemistry 37, 1861 (1964); solid solutions of LaB.sub.6 and an isomorphous alkaline earth hexaboride such as BaB.sub.6 or SrB.sub.6, United Kingdom Pat. No. 1,232,523; and (Y.sub.1-x Eu.sub.x)B.sub.6, O&lt;x&lt;1, U.S. Pat. No. 3,932,314. While promising, these approaches have not been completely successful. Long term chemical or physical stability of these systems is not assured because one component, e.g., Na, is relatively volatile or surface conversion of one hexaboride system to another system, e.g., YB.sub.6 to YB.sub.4, is possible.